1. Field of the Invention
The present invention relates to a dipole antenna which includes a plurality of generally parallel metal wiring lines as a base structure and a plurality of identical or similar unit circuits arranged in a row along the extending direction of the metal wiring lines and connected to one another. The present invention also relates to a planar antenna and a loop antenna obtained through modification of such a dipole antenna.
The present invention is considerably useful for reducing the size of an antenna having a periodic structure.
2. Description of the Related Art
FIG. 21 shows the structure of a dipole antenna AN11 of reduced size according to a conventional technique. A metal wiring line p11, which has at its center a feeding portion F composed of two feeding points FL and FR, is disposed on one surface of a dielectric substrate d11. In this structure, due to the wavelength-shortening effect of the dielectric substrate d11, the antenna length of the dipole antenna AN11; i.e., the length of the metal wiring line p11, can be shortened, and the antenna resonates when its antenna length becomes αd·|n|/2 times the free-space wavelength of electromagnetic waves to be handled, where αd is the shortening ratio for the antenna length, and assumes a value between 0 and 1 depending on the dielectric constant and the z-direction thickness of the dielectric substrate. Further, n is a natural number corresponding to each resonation mode, and the mode of n=1 is typically used, because the antenna length can be shortened to the greatest degree.
Another known antenna in which inductors and capacitors are disposed periodically so as to utilize the left-hand-system phenomenon in which the direction of the group velocity becomes opposite that of the phase velocity in propagation of electromagnetic waves is described in L. Lei, C. Caloz, T. Ito, et. al., “Dominant mode leaky wave antenna with backfire to endfire scanning capability,” Electron. Lett., vol. 38, no. 23, pp. 1414-1416, November 2002. This literature discloses other application forms regarding the left-hand-system phenomenon (an invention relating to an improved leaky wave antenna) and the operation principle of the antenna regarding the left-hand-system.
The first-mentioned conventional technique has a drawback as follows. Since the antenna length shortening ratio αd is determined by the dielectric constant and thickness of an individual dielectric substrate, setting the shortening ratio αd to an arbitrary value is not necessarily easy.
At 100 MHz, for example, a half-wavelength dipole (n=1) placed in a free space has a length of 1.5 m. In the case where the antennal length of such a half-wavelength dipole is shortened in accordance with the conventional technique, a dielectric substrate having a high dielectric constant and a large thickness of about 10 to 50 cm becomes necessary.
Such a dielectric substrate is difficult to manufacture at low cost, and such an antenna can be installed only at limited locations.